Step 1: Make it illegal (if it isn't already) to remove pollution / noise reduction features from cars.
Step 2: Enforce a yearly or bi-yearly tech inspection that checks the pollution / noise against the standard the car was made for (so old cars can still run on older pollution standards)
There, problem solved. Seems like government over-reach? Well, that's how it is in several developed countries in Europe.
That doesn't stop 100% of people from modifying their cars, but it'll quickly become very very niche, because consequences for doing that are too high for most enthusiasts. The main consequence are (1) if police checks and the pollution / noise reduction features are not there, you can be ordered to get your car back to stock in quick order, or in the worst case the car will be destroyed, and (2) in case of a crash, your insurance drops you because your car is not road-legal.
As with all things, there must be a political will to do that, and some people will fight it very hard.
Step 2: Enforce a yearly or bi-yearly tech inspection that checks the pollution / noise against the standard the car was made for (so old cars can still run on older pollution standards)
There, problem solved. Seems like government over-reach? Well, that's how it is in several developed countries in Europe.
That doesn't stop 100% of people from modifying their cars, but it'll quickly become very very niche, because consequences for doing that are too high for most enthusiasts. The main consequence are (1) if police checks and the pollution / noise reduction features are not there, you can be ordered to get your car back to stock in quick order, or in the worst case the car will be destroyed, and (2) in case of a crash, your insurance drops you because your car is not road-legal.
As with all things, there must be a political will to do that, and some people will fight it very hard.